UMD-led Study Finds Climate Answers in Trees—Particularly Big Ones

A new University of Maryland-led study by an international team of scientists paints a vivid image of how forests developed over centuries and contribute to Earth’s carbon balance—a crucial component to maintaining a steady global climate.

The study led by Postdoctoral Associate Ann Raiho of the Earth System Science Interdisciplinary Center (ESSIC) reconstructed the natural pace and pattern of carbon storage in forests of the Midwestern United States over millennia. Published today in the journal Science, the findings have the potential to shift ongoing debates about how landscapes can be managed to maximize carbon storage while meeting conservation goals.

Plants breathe in carbon from the atmosphere and store it in their leaves, branches, trunks and roots. While this “woody biomass” contains one of the largest pools of terrestrial carbon, changes in the magnitude of woody biomass over millennia are poorly known, with most direct observations of vegetation biomass spanning no more than a few decades.

Because trees grow very slowly, this lack of data leads to a substantial knowledge gap. In the absence of empirical data, scientists make assumptions that lead to uncertainties about long-term carbon sink and projections of the future carbon-climate system.

“We found that the forests in the Midwestern U.S. were expanding and getting bigger over the last 10,000 years,” said Raiho, a postdoctoral associate at ESSIC. “This tells us that the prehistoric baseline for understanding forests was wrong and that it's important from a carbon sequestration perspective to preserve trees that grow larger and live longer.”

For the study, the team developed ReFAB (Reconstructing Forest Aboveground Biomass), which models above-ground woody biomass based on fossil pollen discovered in sediments, then reconstructed changes in woody biomass across a 600,000-plus kilometer area over the last 10,000 years.

The researchers found that after an initial postglacial decline, woody biomass nearly doubled during the last 8,000 years. This result differs substantially from prior reconstructions of forest biomass in eastern Canada, which could be due to forest species differences between regions. Previous studies also used simpler models that did not account for uncertainties in the data and found results that indicated little or no change in biomass over the last 6,000 years. ReFAB allows researchers to zoom in to a finer scale, uncovering trends that were previously hidden.

“We found that the ecology of forests matters for understanding the carbon cycle,” Raiho said. “The steady accumulation of carbon was driven by two separate ecological responses to regionally changing climate: the spread of forested biomes and the population expansion of high-biomass tree species within forests.”

But the woody biomass that took millennia to accumulate was destroyed in less than two centuries. Industrial-era logging and agriculture caused woody biomass in the region to decline at a rate more than 10 times greater than at any other time over the last 10,000 years.

This discovery could change the way that forests are managed to mitigate the effects of climate change. Storage of biomass in the region was driven by the population expansion of high-biomass—or large—tree species such as the Eastern hemlock and American beech. Once these species were established, high biomass forests were sustained regionally for millennia. This reconstruction confirms arguments that high-biomass species in old forests play important roles in carbon storage and should be preserved.

“Forest management should emphasize sustaining populations of large trees,” Raiho said. “This has the potential to emulate the natural carbon sequestration processes and ultimately extend the timescales and magnitude at which terrestrial ecosystems will continue to buffer climate change by acting as a carbon sink.”

The study drew on NASA’s Global Ecosystem Dynamics Investigation (GEDI), another UMD-led project that provides high-resolution maps of the 3D structure of forests around the world. GEDI will expand existing knowledge about biomass magnitude, structure and density. With this wealth of information, researchers will be able to better predict the future of forests.

Raiho and her team plan to use this reconstruction data to improve the simulation models used by the Intergovernmental Panel on Climate Change to better understand how climate change will impact Earth and its ecosystems.

In addition to Raiho, this study included researchers from the University of Notre Dame, University of California, Berkeley, University of Calgary and the U.S. Geological Survey.